Abstract
This thesis is an investigation of metasomatism developed at the contacts of the Pounamu Ultramafics in the Turiwhate Survey District.
The lithologies in the Turiwhate Survey District are characterized by a regional schistosity, sub-parallel in most cases, to the original lithological layering. In the quartzofeldspathic schists the earliest recognised schistosity, S₁, is a result of the F₁ deformation period of tight isoclinal folding. Departure from true parallelism of S₀ and S₁ planes produces an L₁ lineation as the line of intersection. L₁ trends north-east and plunges steeply to the east, and is collinear with observed axes of F₁ mesoscopic folds. Although original sedimentary structures are commonly preserved, S₀ structures are obliterated in hinge zones of F₁ mesoscopic folds.
F₁ structural elements are clearly represented in the ultramafic bodies indicating that emplacement of the ultramafic occurred prior to F₁ deformation. The contacts between the ultramafic bodies and the surrounding schists are parallel to S₀ surfaces. The McArthur Crags-Griffin Range ultramafic pods outline a steeply plunging F₁ macroscopic fold with the fold hinge exposed on the southern extremity of McArthur Crags.
Broad, open F₂ post-metamorphic folds occur along slightly plunging axes. Mesoscopic and microscopic expression of F₂ deformation is given by the growth of late-stage chlorite plates on S₂ surfaces across the S₁ foliation.
F₃ deformation occurred in response to high-angle east striking faults which are associated with movement along the Alpine Fault. On a mesoscopic and microscopic scale F₃ structures are typically kink-folds and are restricted to the rocks adjacent to the east striking fault planes. The formation of gem quality nephrite through the production of a highly tectonized, non-oriented fabric is due to F₃ deformation of metasomatic tremolite.
The Pounamu bodies are comprised of a core of generally low alumina antigorite with metamorphic olivine (Fo₉₄-Fo₉₈) developed only in the larger bodies. The antigorite-olivine core is surrounded concentrically by one of three metasomatic zonation sequences, the normal zonation sequence, the schist-poor zonation sequence or the metasomatic marble sequence. Each of the sequences exhibits a marked trend towards monomineralism or bimineralism.
The normal zonation sequence consists of an inner zone of antigorite:magnesite, surrounded successively by a talc:magnesite zone, monomineralic talc zone, and a monomineralic zone of interwoven tremolite. A zone of massive magnesian chlorite with accessory prismatic sphene represents the innermost, schist-derived metasomatic zone. The original schist:ultramafic border is placed at the tremolite and chlorite zone contact on the basis of a pronounced trace element discontinuity. On the schist side of the chlorite zone, the metasomatic muscovite zone consists of non-oriented phengitic muscovite and phlogopitic biotite overprinting an earlier S₁ schistosity preserved by the aligned metamorphic epidote grains. Podiform albitites occur discontinuously between the chlorite and muscovite zones.
The schist-poor zonation sequence is formed in regions of low schist to ultramafite ratio, and consists of an antigorite:tremolite zone; an antigorite:tremolite:chlorite zone; a tremolite:chlorite zone and a chlorite zone.
The metasomatic marble sequence is formed in the ultramafite near the schist contact by replacement of antigorite by idioblastic tremolite followed by the pseudomorphic replacement of antigorite and tremolite by dolomite. The replacement sequence gives rise to bands of calcite and dolomite surrounded by a shell of tremolite crystals within an antigorite matrix.
Un-metasomatized quartzofeldspathic schists surrounding the ultramafite maintain mineral assemblages diagnostic of the garnet zone of epidote-amphibolite metamorphic terrains, i.e. quartz, peristerite plagioclases, biotite, phengitic muscovite, epidote, almandine garnet, and chlorite with apatite, sphene, rutile, and sulphides in accessory proportions. Almandine garnet with a high grossular component and clinozoistic epidote are concentrated adjacent to the metasomatic zones.
Greenschists contain the assemblage; peristerite plagioclases, biotite, chlorite, epidote, hornblende, and almandine garnet with accessory proportions of magnetite, sphene and sulfides. Metacherts and marbles occur as thin lenses within greenschist lithologies.
An experimental investigation of metasomatism was carried out in cold-seal apparatus over periods extending to 40 days at 450°C and 2Kb. In several runs, oxalic acid was used to provide an internal buffer and as a source for the production of CO₂. The gas phase was analysed by mass spectrometry. The absence of CH₄ indicates that the experimental metasomatic event occurred under conditions of relatively high P(O₂) enabling the gas phase to be defined in terms of an ideal binary X(H₂O)-X(CO₂) system.
Petrographic and electron microprobe analyses of the experimental runs indicate component migration trends in order as follows; CO₂, SiO₂ and CaO migrate from the schist into the ultramafite; MgO migrates from the ultramafite into the schist; Na₂O and K₂O migrate from the schist adjacent to the ultramafite to the ultramafite further into the schist; Al₂O₃ and TiO₂ remain in their original configurations. The component migration scheme leads to the formation of reaction zones within both the schist and the ultramafite. In the schist the reaction zone consists of fine grained chlorite replacing the other schist specie by the dissolution of quartz, albite, epidote, muscovite, biotite, and garnet. Magnesite and talc are the reaction products within the ultramafite formed by the breakdown of antigorite upon the introduction of CO₂ and SiO₂.
In conjunction with the experimental and petrographic data, a thermodynamic approach is utilized to produce a comprehensive scheme of component behaviour during the metasomatic event. The metasomatic zines formed by diffusion of components in a direction such that energy differences caused by the juxtaposition of the schist and ultramafic bodies were minimized. The reaction zones are assemblages in local equilibrium with each adjacent assemblage. Diffusion occurs down chemical potential gradients for each diffusing component. The incompatibility of chemical potential gradients defined solely by compositional gradients is illustrated, and a method for delineating and demonstrating the behaviour of the chemical potentials of diffusing components in complex systems is formulated. The method utilizes molar free energy:composition plots to define behavioural trends of perfectly mobile and inert components. As an illustration, the properties of molar free energy:composition plots are applied to the case of SiO₂ diffusion in the normal metasomatic zonation sequence. The Theories derived from the examination of SiO₂ diffusion are then utilized in a qualitative manner to define diffusion phenomena in the normal metasomatic sequence, the schist-poor sequence and the metasomatic marble sequence.
Relative component mobilities are examined and an order of increasing mobility among the system components is developed. The rate of diffusion is shown to be time and distance dependent and the existence of a theoretical point beyond which the rate of diffusion is infinitesimal is shown.
Simple mass-balance calculations combined with garnet and epidote analyses indicate the existence of high concentrations of CaO and Al₂O₃ prior to the latest metasomatic event. The high concentrations of these components are postulated to be due to the development of rodingite assemblages during serpentinization of the ultramafite in its present quasi-stratigraphic position. Diffusion during the later metamorphic event redistributed the rodingite components.
With the above data, an approximate temporal sequence of serpentinization, deformation, metamorphism and metasomatism, and post metamorphic deformation is delineated.